Various voltage level metal-oxide-semiconductor (MOS) transistors, such as a CMOS transistor, a drain-extended MOS (DEMOS) transistor and a lateral-diffused MOS (LDMOS) transistor, may be selected in accordance with different applications. In power applications, a drain voltage is generally higher than a gate voltage. In order to sustain the drain voltage, the DEMOS transistor or the LDMOS transistor having higher breakdown voltage is a better choice. However, Rdson (drain-to-source on-state resistance) of the DEMOS transistor or the LDMOS transistor is significantly greater than that of the CMOS transistor. It is noted that to raise breakdown voltage and to reduce Rdson are conflicted with each other. Accordingly, there is a need for an improved transistor structure having low Rdson and high breakdown voltage which is beneficial to the power applications.